83

Does Plant Litter Become More Recalcitrant Under ElevatedAtmospheric CO2 Levels?
B. Mohan KUMAR,1* Kikuo HAIBARA2 and Hiroto TODA2
1

College of Forestry, Kerala Agricultural University,
KAU PO, Thrissur 680656, Kerala, Indiae-mail: bmkumar53@yahoo.co.uk
2
Tokyo University of Agriculture and Technology (TUAT),
3-5-8 Saiwaicho, Fuchu-shi, Tokyo 183-8509, Japane-mail: kikuo@cc.tuat.ac.jp (*author for correspondence)

Abstract
Increases in atmospheric CO2 concentration exert several positive and negative effects on litter
dynamics, but the net impact of climate change on litter decomposition and the carbon balance of theterrestrial ecosystem is yet unclear. The objective of this review is to summarize the current state ofknowledge on litter dynamics under elevated CO2 levels with a view to using it as a basis for mitigating
the negative effects of rising anthropogenic CO2 emissions. To understand the biogeochemical processes
in response to elevated CO2 levels, we propose a conceptual model illustrating the interactive effects ofrising CO2 levels on litter dynamics, which assumes that the expected positive effects on photosynthesis
will be translated into higher detritus production. The general dilution of nitrogen within plant tissues
grown under elevated CO2 and the consequent N constraints on microbes, however, may have a strong
feedback effect on photosynthetic efficiency through depressed nutrient mineralization. Again, aconcomitant rise in the atmospheric temperature may accelerate the process of litter decomposition.
Significant regional variations in litter production and decomposition rates are also probable in view of
the changes in precipitation intensity/patterns and/or biodiversity changes associated with global warming. Gains in litter accumulation on the forest floor under elevated CO2 levels may lead to increased carbon sequestration and could partially offset the negative effects of anthropogenic CO2 emissions into theatmosphere.
Key words: Carbon accumulation, decomposers, global warming, litter dynamics,
net primary productivity, substrate quality

1. Introduction
The concentration of CO2 in the earth’s atmosphere
has increased since the beginning of slash-and-burn
agriculture and in particular after the beginning of theindustrial period. The current average annual increaseis about 1.5 µL L-1, with a predicted doubling of preindustrial concentrations by the end of the 21st century(IPCC, 2001). In 1995, the Intergovernmental Panelon Climate Change produced a ‘business as usual’scenario which foresaw annual global emissions of
CO2 rising from 7.4 Pg C to 26 Pg C in the year 2100
along with significant increases in other green housegases (GHGs) including methane, nitrous oxide,hydrofluorocarbons, perfluorocarbons and sulphur
hexafluoride (IPCC, 1996). The most important consequence of changing atmospheric composition is apronounced warming of the global climate. Boreholemeasurements worldwide indicate a global surface
Global Environmental Research9(1)/2005: 83-91
printed in Japan

warming of around 1°C during the last 500 years, with
about half of this warming occurring in the 20th century. Climate experts also predict a global surface
warming of 0.6 to 2.5°C in the next 50 years and 1.4
to 5.8°C by the end of the 21st century, with significant regional variations (IPCC, 1996).
Changes in atmospheric composition and the resultant increases in global temperatures are also associated with changes in precipitation intensity and/or distribution. With rising temperatures, global precipitation is expected to increase a priori, due to thegreater rates of evaporation of sea surface water. Consistent with this, precipitation increased (2% to 20%)
over land areas at high latitudes of the Northern Hemisphere between 1900 and 1994. However, a decreasein precipitation occurred gradually after the 1960s
over the subtropics and the tropics from Africa toIndonesia (IPCC, 2001). Simulation models also
project a climate change-induced increase in annual©2005 AIRIES

84

B. Mohan KUMAR et al.

precipitation in high and mid latitudes and mostequatorial regions but a general decrease in thesubtropics (Carter et al., 2000). Lal et al. (2001),
using a coupled atmosphere-ocean general circulation
model (A-O GCM), predicted a 5% to 25% decline inwinter rainfall and 10% to 15% increase in monsoon
rainfall over South Asia during the 2080s. If thesepredictions are true, there may be increased drought
occurrence during the dry months and more intense
rainfall spells during the wet season, particularly inthe tropics.
During recent years concern has also been growing
among scientists and the general public about thepossible impacts of future climate change on terrestrial ecosystems, especially with respect to plant
growth, changes in biodiversity, distribution patternsof naturally growing tree species and the overall
impact on carbon storage in the biosphere (Rasmussen
et al., 2002). With plant litter being both a sink as well
as source of carbon and mineral nutrients, there has
been profound interest in litter dynamics. Although it
has been emphasized that changes in litter chemistry(reduction in litter N concentration and a concomitant
increase in lignin levels) in an atmosphere withelevated CO2 level, will reduce decomposition rates
and release of N from organic materials (Norby et al.,
2001; Lindroth et al., 2001), the ecological effects ofincreased CO2 and global warming on litter dynamics
are more complex and difficult to predict in view ofthe following:• Plant responses (e.g., photosynthesis, respiration,
litter chemistry, etc.) vary with species and growth
factors (Cuelemans, 1997; Mooney et al., 1999).
• Individual processes in ecosystem biogeochemistry
may oppose each other; hence laboratory andgreenhouse experiments, often with single species
and young seedlings cannot be directly extrapolated to mature ecosystems (Cuelemans, 1997; Van
Breemen et al., 1998; Mooney et al., 1999).
• Anthropogenic N additions (Bobbink et al., 1998)
and the close link between C and N cycles and thepotential for feedback among them (Tateno &Chapin, 1997) confound the effects of rising CO2
levels.
• Few studies have compared the interactive effectsof various GHGs (e.g., CO2, O3, etc.) and risingtemperature under altered soil moisture/nutrient
regimes on ecosystem-level responses; and virtually none from the tropics. Recent free air CO2 enrichment (FACE) studies in temperate regions (e.g.,
Kasurinen et al., 1999; Karnosky et al., 1999;
Olszyk et al., 2001) and comparative analyses of
elevated CO2 on ecosystem processes (e.g.,
Sowerby et al., 2000; Norby et al., 2001;
Seneviratne, 2000; Ross et al., 2003), however,
have advanced knowledge in this respect; but
clearly, more studies are needed for otherecosystems, and on the concomitant effects ofrising temperature/altered soil moisture regimes

and changing decomposer populations on litter
dynamics.
In this paper we address questions relating to how
litter dynamics and the associated nutrient turnoverprocesses will be affected under changing atmosphericcomposition. Although previous reviewers have addressed aspects such as litter chemistry and/or soil
microbial activity (e.g., Zak et al., 1993; Seneviratne,
2000; Norby et al., 2001), the larger issue of how
global warming would impact litter dynamics andnutrient cycling remains largely unanswered. The
objective here is to summarize and analyze the recent
trends in ecosystem research pertaining to litter
dynamics under elevated CO2 levels with a view toproviding some new insights and indicating gaps in
the existing knowledge base.

2. Climate-substrate-biota-interactive Litter
Dynamics ModelIn this article, we present a comprehensive view on
litter dynamics, focusing on a conceptual model forinteractions among environmental parameters, litter
production, biochemical quality of litter and decomposer communities (Fig. 1). There are many statevariables and driving functions in this model (theboxes and arrows, respectively), implying an intricate
relationship characterised by positive, negative or neutral (‘+’, ‘−’ or ‘0’) feedback effects. The model predicts that litter production, decay and mineralization
are deterministic outcomes of these interactions. Thatis, the linkages among climate, floristic attributes,
substrate quality and biotic subsystems are major
drivers of decay dynamics. The ensuing sections explain how each of these state variables/driving functions is altered under elevated CO2 levels.
2.1 Net primary productivity
A large volume of literature exists on the impact ofelevated CO2 levels on photosynthesis and net primary
productivity (NPP). Most of these could be rationalized based on two experimental paradigms, viz., the‘CO2-fertilization’ and the ‘litter quality’ (negative
feedback) hypotheses. The general consensus amongproponents of the CO2 fertilization hypothesis is thatelevated CO2 stimulates photosynthesis, and consequently NPP. Consistent with this, several workers
reported increased plant growth associated with increased photosynthetic rates and decreased root respiration in response to elevated CO2 levels (Koch &Mooney, 1996; Johnson et al., 1998; Curtis & Wang,
1998; Mingkui & Woodard, 1998; Luan et al., 1999;
Olszyk et al., 2001).
The consequent increase in forest productivity,
notably in the mid-latitudes (Fan et al., 1998) may
increase labile C inputs into the soils (Finzi et al.,2001), which in turn, may sustain NPP and/or stimulate soil carbon sequestration. Yet another positive
impact of elevated CO2 is reduced stomatal

Litter Decomposition Under Elevated CO2 Levels

85

Fig. 1 Conceptual model of the feedback/feed forward effects of rising atmospheric
CO2 levels on litter dynamics (symbols: ‘+’, ‘–’ and ‘0’ indicate positive,
negative and neutral effects)

conductance of plants which results in higher water
use efficiency. At the community level, this often
results in decreased stand transpiration and higher soil
water content (Körner, 2000) and can potentiallystimulate photosynthetic production in water-limited
ecosystems.
Conversely, the ‘litter quality hypothesis’(McGuire et al., 1995; Norby et al., 2001) implies thatchanges in litter chemistry owing to CO2 enrichment
and the consequent slower decomposition, may have a‘seesaw’ effect on soil N pool. Because plants arecommonly N-limited in terrestrial ecosystems, andany tendency for increased CO2 to decrease N availability may suppress primary productivity (Cotrufo etal., 1998a; Norby & Cotrufo, 1998; Fahey et al.,1998). Likewise, many authors reported that the effects of elevated CO2 levels on NPP were not significant (Pérez-Soba et al., 1995; Lippert et al., 1997), orthat the initial increase in NPP will not be sustained
due to nutrient limitations (Comins & McMurtrie,
1993; Luo & Reynolds, 1999). Moreover, simulation
experiments suggest that the initial increase in forest
growth and NPP in response to doubling of atmospheric CO2 concentrations may decline to originallevels (e.g., Comins & McMurtrie, 1993; Luo &Reynolds, 1999).
Neither model, however, considers the concomitant effects of rising temperature, O3 and other GHGs.
Clearly, the effects of elevated atmospheric CO2 on
ecosystems will not occur in isolation, but in the context of multiple environmental stresses (Grulke, 1999;
McBride & Miller, 1999; Kumar, 2003) including
herbivory (Coley, 1998). Regarding temperature effects, which are crucial and the focal theme of this
discussion, warmer summer air temperatures due toglobal warming may be beneficial in the temperate
latitudes, where the length of the growing season andfrost-free period would increase. Nonetheless, warmer
temperatures exert negative effects in those regions
where temperature and water stress limit NPP, andmay cause early senescence of leaves. In the event of
reduced summer rainfall in the tropics, as many GCM
projections indicate, the photosynthetic efficiency will
be further curtailed. Thus, it is likely that in a ‘business as usual’ scenario, the net ecosystem productivity(NEP) in most parts of the tropics and subtropics will
be depressed, whereas there may be significant gains
in the temperate zones. The predicted shifts in climate
zones with global warming (IPCC, 2001), however,complicate this pattern.
2.2 Dinitrogen fixation
Nitrogen fixation is an energy-requiring process
and derives its energy requirement from the oxidative
metabolic pathway (Krebs cycle). The C skeletons
required for converting N, fixed in the root nodules,
into amino acids are also drawn from this pathway.
Thus an increase in photosynthetic efficiency according to the CO2-fertilization model, following CO2
enrichment, is likely to favour the biological nitrogen
fixation (BNF) process. Conversely, any negative
effects on photosynthesis owing to water and nutrient
deficits may adversely affect it. The effects of rising
CO2 levels on BNF are, however, more complex, as
concomitant changes in soil nitrogen release andenvironmental stresses exert positive or negative
feedback controls on the photosynthetic/respiratory
pathways and also on the activity of nitrogen fixing
organisms. Experimental evidence available in thisrespect is not only scanty but also inconsistent. For
example, Olszyk et al. (2001) found no significant
effects of elevated CO2 levels on autotrophic N fixation rates in a ponderosa pine ecosystem, whereas
Temperton et al. (2003) reported consistently higher

86

B. Mohan KUMAR et al.

nitrogenase activity in Alnus glutinosa grown underelevated CO2 levels. Such variations in experimental
results probably imply differences owing to the organisms involved and/or the method of assay used.
Many authors (Rygiewicz et al., 2000; Staddon etal., 1999; Olszyk et al., 2001) also noted that elevated
CO2 increases mycorrhizal colonization, but in certain
cases the response was transitory and dependent on P
nutrition (Walker et al., 1995). Overall, as the forests
grow under elevated CO2, the fluxes of N and P in
vegetation, microbes and soils are expected to increase, and a positive feedback to develop between
elevated CO2, NPP and plant nutrient availability.
2.3 Litterfall production
The probable effects of elevated CO2 levels andthe consequent atmospheric warming on litterfall production can be summarized as follows. On the positiveside, a large body of literature suggests that elevated
CO2 may increase plant photosynthesis, net primary
production and thus, litter production. In consonancewith this, in the Duke Forest FACE study, significant
increases in loblolly pine leaf litterfall mass wereobserved (Allen et al., 2000; Finzi et al., 2001). In
view of the limited nature and range of the experimental studies reported –mostly from the temperate zones–
it is perhaps difficult to draw valid generalizations on
the effects on litterfall production under increased
CO2 levels across the globe. Nevertheless, in view ofthe overwhelmingly positive effects of CO2 enrichment on plant production owing to the warmer
temperature in the temperate regions, increased N(biological and anthropogenic) and labile C inputs, it
seems reasonable to assume that litter inputs may increase in the temperate ecosystems, but the picture in
the tropics is still obscure.
2.4 Litter chemistry
The central hypothesis of litter dynamics is that N
and lignin control the decay process (Melillo et al.,
1982; Kumar & Deepu, 1992; Cadisch & Giller, 1997
and many others). Lignin is recalcitrant to enzyme
degradation, and its intimate association with cellulose
fibers results in masking of a large fraction of carbohydrate, which otherwise would be accessible tomicrobes (Gessner & Chauvet, 1994). Likewise, polyphenolics (e.g., tannins) form resistant complexes withproteins (Hättenschwiler & Vitousek, 2000), directly
inhibiting microbial growth and activity (Swift et al.,1979). Hence, the lignin and poyphenol contents oflitter are often regarded as inverse indexes of carbon
availability to decomposers, and thereby decay rates(McClaugherty & Berg, 1987; Berg, 2000; Kumar &Goh, 2000).
It has generally been assumed that rising atmospheric CO2 concentrations will increase the concentration of carbon-based secondary or structuralcompounds in plant tissues. Increased biomass productivity also may lead to a concomitant reduction in

N concentrations (dilution effect). Consistent with this,several workers (see reviews by Koricheva et al.,
1998; Norby et al., 2001) noted that CO2 enrichment
increased levels of tannins and starch in litter, but decreased foliar levels of N, and the magnitude of suchvariations were dependent on both plant species andresource availability (e.g., light, nutrients).
However, lack of consistent evidence with respect
to the changes in litter chemistry obscures this pattern.
For instance, in certain cases, starch levels in CO2 enriched foliage showed no consistent increase overthose in unenriched foliage (Peñuelas & Estiarte,1998). Some authors also observed that chemicalcomposition of green leaves and leaf litter were
unaffected by elevated CO2 (e.g., Finzi et al., 2001;
Olszyk et al., 2001). Naturally senesced litter oftenchanges in C/N as a result of variations in CO2 levels(Norby et al., 2001). Differences among species werealso prominent in some cases. For example, Lindrothet al. (2001) found that condensed tannins for carbonbased metabolites derived from the shikimic acid
pathway increased in response to enriched atmospheric CO2 in birch (Betula papyrifera) leaves, butthere was little response in aspen (Populus tremuloides).
In addition, studies on the impact of elevated CO2
on litter chemistry have excluded the coarse litter fraction, which constitutes a significant part of forest litter,and may be of interest in the context of carbon
accumulation in terrestrial ecosystems (Mackensen &Bauhus, 1999). Presumably, the lignin/polyphenol
contents of coarse fractions may show a greater degree of variability in response to elevated CO2 thanfoliage litter, but to our knowledge, no previous studies have examined the effects of rising atmospheric
CO2 levels on either the chemical composition or
decomposition rates of coarse litter.
2.5 Resorption efficiency
Based on experimental evidence on the chemistryof fresh foliage, it can be deduced that lower N levels
will be characteristic of leaf litter under elevated CO2
levels. It is, however, debatable whether the N of fresh
leaves accurately predicts the N content of litter or,indeed, its subsequent decomposition rate (Norby etal., 2001) – because resorption plays a pivotal role indetermining the N concentration of leaf litter, andthereby the soil N pool. An increase in resorption prior
to senescence generally tends to decrease the N concentration of litter and increase the residence time oflitter.
Resorption efficiency in senescing leaves is presumably under environmental control; i.e., more efficient under conditions of CO2 enrichment (Cotrufo etal., 1998b; Coûteaux et al., 1999) and less efficientunder high soil fertility (Jamaludheen & Kumar, 1999).
Here again, however, scientific opinion is divided, justas in many other aspects relating to CO2 enrichment
and biogeochemistry. Some authors (e.g., Aerts &Litter Decomposition Under Elevated CO2 Levels

Chapin, 2000; Heerwaarden et al., 2003) suggest that
resorption efficiency is not very responsive to changes
in nutrient supply, while others (Lindroth et al., 2001;
Finzi et al., 2001) argue that resorption efficiency is
not affected by CO2 enrichment.
Yet another complicating factor is that, in many
ecosystems N availability has increased during recent
decades due to enhanced atmospheric N deposition(Bobbink et al., 1998). It can, therefore, be rationalized that if global warming enhances soil N concentrations, it will probably result in lower resorption
efficiency, and therefore, more N available formicrobial and plant growth. As a result, if resorption
efficiencies decline, then the effects of CO2 on N
levels in green leaves may not carry over to leaf litter;i.e., the ‘litter quality hypothesis’ will not be
supported. Resorption, however involves changes notjust in N, but in dry matter as well, and the resulting N
concentration in litter is determined by changes inboth N content and dry matter content (Norby et al.,2001). Also, more efficient retransolation is expectedin the twigs and branches in view of their greater
longevity, not to mention the improbability of their‘early senescence,’ as in the case of foliage.
2.6 Litter decay
While changes in litter chemistry in an atmosphereof elevated CO2 (i.e., lower N and higher lignin/
polyphenol concentrations in foliage of plants) will
decelerate decomposition, increased atmospheric N
inputs may offset any potential negative effects onlitter decay owing to intrinsically lower litter N concentrations. Although not strictly comparable, Norby
et al. (2001) observed that across a database covering
33 herbaceous plants and 48 woody species, neither
the mass loss nor respiration rates from leaf litter produced under elevated CO2 showed any consistent pattern or differences. Likewise, Finzi et al. (2001) reported that the rate of mass loss was unaffected by‘litter type’ (i.e., whether the litter was produced underambient or elevated CO2 levels); and ‘site’ of decomposition (i.e., whether the litter was decomposed in theambient or elevated CO2 plots). It should be noted thatthese studies did not consider variations in soil moisture availability and/or decomposer populations,which usually co-vary with atmospheric CO2 levels in
the natural environment. Yet, it can be argued that the
net effect of rising CO2 levels on decay rates owing tochanges in microbial N availability under ‘steadystate’ conditions (no concomitant variations intemperature and moisture regimes, as predicted by theclimate models) is modest. Having stated, there could
still be potential effects of increasing lignin/polyphenol content, in particular in the coarse litter fractions that is likely to contribute more to litterfall
production as NPP increases.
2.7 Soil moisture and temperature
The direct effects of abiotic factors such as tem-87

perature and moisture on the decomposition process
are well documented. That is, within a threshold,microbial activity increases exponentially with temperature, and as a result, high temperatures are likely
to hasten decomposition (Kumar & Goh, 2000;
Seneviratne, 2000). However, the temperature effecton the decay process is moderated by litter/soil moisture content. Potential moisture limitations as a resultof global warming in the tropics can restrict microbialactivity of soils. Many authors have also reported
higher relative loss of litter mass during the rainy season than during the dry season (Facelli & Pickett,
1991; Thönnissen et al., 2000; Vucetich et al., 2000),
implying a proximate control of summer moisture on
litter decomposition. Therefore, if the global warming
process reduces summer precipitation rates in the tropics/subtropics, as predicted in the GCMs, then litter
decay will in all probability be adversely affected.
2.8 Species diversity and litter decomposition
2.8.1 Microbiological changes
Theoretically, rates of ecosystem processes increase linearly with species richness if all species contribute substantially and in unique ways to a given
process (Johnson et al., 1996). By extension, microbial and meso-faunal richness and diversity can leadto increased decomposition of litter and organic matter.
Changes in the global environment, however, havealtered biological diversity at all organizational levels,
ranging from genetic diversity within populations to
the diversity of ecosystems (Chapin et al., 2000), andthe effects are likely to be more potent in the future.
Loss of species diversity has functional consequencesin terms of litter decay because the number and kindsof species determine the organizational traits that
influence any ecosystem process (Hooper & Vitousek,
1997). Previous investigations (Salonius, 1981) showedthat microbial species richness increased organic
mater decomposition, although others found no predictable relationships between number of organisms
and the decay rates (e.g., Wardle et al., 1997; Chihara
et al., 2000). Chapin et al. (2000) explained suchvariations in experimental results to be on account of
the differential sensitivity of ecosystem processes tospecies richness vis a vis other components of diversity such as evenness, composition or interactions, orto the strong dependence of ecosystem processes on
one or a few species.
Microbial responses to elevated CO2 in complex
natural ecosystems are also less understood(Kampichler et al., 1998). Some workers (e.g.,
Lamborg et al., 1983; O’Neill, 1994) thought that
direct effects of elevated CO2 concentrations on soilorganisms were improbable because CO2 concentrations in soils were already 10 to 50 times greater thanin the atmosphere. Zak et al. (2000), however,concluded that the response of soil microorganisms toelevated CO2 is highly variable. Consistent with this,88

B. Mohan KUMAR et al.

published results show a range of responses from nomarked increase in the soil microbial biomass withelevated CO2 (Runion et al., 1994; Kampichler et al.,1998) to significantly greater levels of microbial biomass (Diaz et al., 1993; Sowerby et al., 2000) or
activity (Ebersberger et al., 2003). Higher metabolic
activity in the soil was rationalized on account of theextra C entering the soil through greater fine root turnover and higher rhizodeposition (Rogers et al., 1994).
Although changes in microbial communities in response to rising CO2 levels are certain, it is yet unclear
whether such changes will result in increased or decreased decay rates. This is partly because of a lack ofsufficient evidence regarding species changes consequent to global warming and the variable influence ofindividual organisms on the decomposition process.
Also, most information on soil microbial response toelevated CO2 originates from short-term experiments
and/or experiments with disturbed soil (Hu et al.,1999). Extrapolation of these results to mature ecosystems and to longer time scales is rather risky.
2.8.2 Changes in invertebrates and vascularflora
The vast majority of the soil organisms are heterotrophs (e.g., soil invertebrates, such as earthworms,
termites and the microarthropods) and any changes intheir population dynamics will certainly affect organic
matter decomposition (Lavelle et al., 1997). Climatechange has the potential to drastically alter the diversity attributes (richness, evenness) of these organisms,
besides that of higher plants (Thomas et al., 2004).
While changes in the structure of the soil invertebrate
fauna have a direct impact on litter decomposition,changes in the floristic composition exert a countervailing influence on decomposition, through changes
in litter chemistry, relative proportion of different litter
fractions and the amount of root exudates production.
Indeed, a large body of literature indicates that litter/organic matter decomposition is dependent on factors such as origin of the litter (i.e., from C3 or C4
plants; Kemp et al., 1994; Ball & Drake, 1997; Ross
et al., 2003), composition of plant tissues (roots andshoots; Rogers et al., 1994; Johnson et al., 1998;
Allen et al., 2000; Olszyk et al., 2001), relative proportions of coarse and fine litter, and root exudates(Ebersberger et al., 2003). Floristic changes alter mostor all these parameters, and could bring about substantial changes in the population size and diversity andfunctions of food web biota in litter and soil.
2.9 Nitrogen limitation of microbial decomposition
Studies on the effects of increased CO2 on soil microbes have made the implicit assumption that CO2enrichment enhances C input for microbial biomass
and activity, underscoring the premise that soil
microbes are C-limited (Hu et al., 2001). They further
suggest that CO2 enrichment in the atmosphere canexacerbate N constraints on microbes and suppress
microbial use of soil C (Kaye & Hart, 1997; Wang &Bakken, 1997). That is, a decrease in N availability
due to increased CO2, suppresses microbial decomposition, owing to increased competition for N between
plants and microbes. The increased C availability under elevated CO2 would also boost microbial biomass,in turn, leading to nutrient immobilization and constrained plant responses (Diaz et al., 1993). Conversely, Zak et al. (1993) and Ebersberger et al.
(2003) reported increased N mineralization of native
organic matter, resulting in higher N availability toplants. The specific response, however, would depend
on a complex set of factors such as litter quality, theamount of litter inputs and changes in decomposer
populations in response to altered temperature/moisture regimes.

3. Conclusions
Little is known about the interactions between
decomposition dynamics of litter across different
ecosystems and rising levels of atmospheric CO2.
Decomposition drivers such as chemical attributes oflitter, precipitation/temperature regimes and decomposer populations, often vary with one another andwith increasing CO2 levels, making such interactions
intricate. Understanding the nature of these interactions is, nevertheless, crucial to providing a scientificfoundation for evaluating the impacts of climatechange on various aspects of SOM dynamics andnutrient cycling, including the effects of litter/green
manure addition on soil nutrient dynamics. Thus
research (both experimental studies and simulation
modeling to characterize ecosystem responses) isneeded on all fronts and in particular on tropicalecosystems.
Regarding the effects of elevated CO2 levels on
NEP, it is reasonable to assume that NEP will increase
and at least a part of the extra C fixed in response toenhanced CO2 levels will also be allocated below
ground. Concomitant floristic changes, changes inresource availability (e.g., moisture and nutrients)
and/or abiotic factors (e.g., temperature) can, however,alter the NEP and biomass allocation strategies. It is,therefore, difficult to extrapolate the short-term responses to changes in CO2/temperature of individual
ecosystem processes, such as photosynthesis andrespiration to whole ecosystems, and a comprehensive
understanding of the response to global warming remains elusive.
The consequences of CO2 enrichment for plantchemical composition and decomposer communities
are also of immense interest. As regards to the effects
of CO2 enrichment on litter chemistry, there are two
predominant notions. The first is that litter lignin/
polyphenol levels would increase and N levels would
decline under elevated CO2 levels, leading to a negative impact on decomposition. Experimental evidenceLitter Decomposition Under Elevated CO2 Levels

mainly from temperate regions, however, has been
mixed, probably depending on several factors such as
species, experimental conditions and litter chemistry.
The second is, as many workers have indicated, thatthe observed chemical changes in litter have been
largely transitory in nature and there would be little orno change in litter chemistry under elevated CO2 levels in the long run. It can then be argued that decay
rates under elevated CO2 environment would predominantly be determined by factors other thanchemical attributes, such as changes in temperature/moisture levels and/or decomposer populations.
In view of the complex nature of ecosystem processes and inconsistencies in experimental results on
various aspects of atmospheric CO2 enrichment andlitter dynamics, it is perhaps difficult to predictwhether litter decomposition would increase or decrease and what potential effects of this would be onsequestration of the extra carbon fixed in the photosynthetic process. It can be parsimoniously argued thatlitter inputs in temperate regions might increase
following the stimulation of NPP, but decay rates may
decline owing to changes in litter chemistry (e.g.,
lignin/polyphenol concentrations). Consequently, thenet amount of litter and carbon accumulating on theforest floor may increase. As regards to the tropics, thepicture is still hazy, but assuming that environmental
stresses (especially for moisture) would amplify, litter
production may decline. The enhanced CO2 levels
would favor production of high C and low N litter.
This coupled with the strong negative effects of global
warming on soil fauna and flora probably would lower
decay rates. Thus, modest increases in the amount oflitter remaining on the tropical forest floor could be
anticipated, provided ecosystem integrity were maintained.

Acknowledgements
The first author gratefully acknowledges support
received from Japan Society for Promotion of Science(JSPS) in the form of an invitation (short-term)
fellowship at the Forest Ecology Laboratory, Tokyo
University of Agriculture and Technology, Japan
during 2003-2004.
References
Aerts, R. and F.S. Chapin III (2000) The mineral nutrition ofwild plants revisited: a re-valuation of processes and patterns.
Advances in Ecological Research, 30: 1-67.
Allen, A.S., J.A. Andrews, A.C. Finzi, R. Matamala, D.D.
Richter and W.H. Schlesinger (2000) Effects of free-air CO2
enrichment (FACE) on belowground processes in a Pinus
taeda forest. Ecological Applications, 10: 437-448.
Ball, A.S. and B.G. Drake (1997) Short-term decomposition oflitter produced by plants grown in ambient and elevatedatmospheric CO2 concentrations. Global Change Biology, 3:29-35.

89

Berg, B. (2000) Litter decomposition and organic matter turnover in northern forest soils. Forest Ecology and Management, 133:13-22.
Bobbink, R., M. Hornung and J.G.M. Roelofs (1998) The effects of air-borne nitrogen pollutants on species diversity innatural and semi-natural European vegetation. Journal of
Ecology, 86: 717-738.
Cadisch, G. and K.E. Giller, eds., (1997) Driven by nature,
plant litter quality and decomposition. CAB international,
Wallingford.
Carter, T.R., M. Hulme, J.F. Crossley, S. Malyshev, M.G. New,
M.E. Schlesinger and H. Tuomenvirta (2000) Climate
Change in the 21st Century — Interim Characterizations
based on the New IPCC Emissions Scenarios. The Finnish
Environment 433, Finnish Environment Institute, Helsinki,
148 p.
Chapin, F.S. III, E.S. Zavaleta, V.T. Eviner, R.S. Naylor, P.M.
Vitousek, H.L. Reynolds, D.U. Hooper, S. Lavorel, O.E. Sala,
S.E. Hobbie, M.C. Mack and S. Diaz (2000) Consequencesof changing biodiversity. Nature, 405: 234-242.
Chihara, M., N. Oyanagi, H. Toda and K. Haibara (2000)
Effects of soil microbial flora on nitrogen mineralization inforest soil. Forest Resource Environment, 38: 97-196. (inJapanese)
Coley, P. (1998) Possible effects of climate change onplant/herbivore interactions in moist tropical forests.
Climatic Change, 39:455-472.
Comins, H. and R.E. McMurtrie (1993) Long-term response ofnutrient-limited forests to CO2 enrichment; equilibrium behavior of plant-soil models. Ecological Applications, 3:
666-681.
Cotrufo, M.F., B. Berg and W. Kratz (1998a) Increased atmospheric CO2 and litter quality. Environmental Review, 6: 1-12.
Cotrufo, M.F., P. Ineson and A. Scott (1998b) Elevated CO2
reduced the nitrogen concentration of plant tissues. Global
Change Biology, 4: 43-54.
Coûteaux, M.M., C. Kurz, P. Bottner and A. Raschi (1999) Influence of increased atmospheric CO2 concentration on quality of plant material and litter decomposition. Tree Physiology, 19: 301-311.
Cuelemans, R. (1997) Direct impacts of CO2 and temperatureon physiological processes in trees. In: G.M.J. Mohren, K.
Kramer, S. Sabate, eds., Impacts of Global Change on Tree
Physiology and Forest Ecosystems, Forestry Sciences.
Kluwer Academic Publishers, Dordrecht, pp.3-14.
Curtis, P.S. and X. Wang (1998) A meta-analysis of elevated
CO2 effects on woody plant mass, form and physiology.
Oecologia, 113: 299-313.
Diaz, S., J.P. Grime, J. Harris and E. McPherson (1993)
Evidence of a feedback mechanism limiting plant response toelevated carbon dioxide. Nature, 364: 616-617.
Ebersberger, D., P.A. Niklaus and E. Kandeler (2003)
Long-term CO2 enrichment stimulated N-mineralization andenzyme activities in calcareous grassland. Soil Biology andBiochemistry, 35: 965-972.
Facelli, J.M. and S.T.A. Pickett (1991) Plant litter: its dynamics
and effects on plant community structure. Botanical Review,
57: 1-32.
Fahey, T.J., J.J. Battles and G.F. Wilson 1998. Response ofearly successional hardwood forests to changes in nutrient
availability. Ecological Monographs, 68: 183-212.
Fan, S., M. Gloor, J. Mahlman, S. Pacala, J. Sarmiento, T.
Takahashi and P. Tans (1998) A large terrestrial carbon sinkin North America implied by atmospheric and oceaniccarbon dioxide data and models. Science, 282: 442-446.
Finzi, A.C., A.S. Allen, E.H. DeLucia, D.S. Ellsworth and W.H.
Schlesinger (2001) Forest litter production, chemistry, anddecomposition following two years of free-air CO2 enrichment. Ecology, 82: 470-484.

90

B. Mohan KUMAR et al.

Gessner, M.O. and E. Chauvet (1994) Importance of stream
micro fungi in controlling breakdown rates of leaf litter.
Ecology, 75: 1807-1817.
Grulke, N.E. (1999) Physiological responses of ponderosa pine
to gradients of environmental stressors. In: P.R. Miller and
J.R. McBride, eds., Oxidant Air Pollution Impacts in theMontane Forests of Southern California, Springer, New York,
pp.126-163.
Hättenschwiler, S. and P.M. Vitousek (2000) The role of polyphenols in terrestrial ecosystem nutrient cycling. Trends in
Ecology and Evolution, 15: 238-243.
Heerwaarden, L.M., S. Toet and R. Aerts (2003) Nitrogen andphosphorus resorption efficiency in six sub-arctic bog species after 4 years of nitrogen fertilization. Journal of Ecology,
91: 1060-1070.
Hooper, D.U. and P.M. Vitousek (1997) The effects of plantcomposition on ecosystem processes. Science, 277:1302-1305.
Hu, S., M.K. Firestone and F.S. Chapin III. (1999) Soil microbial feedbacks to atmospheric CO2 enrichment. Trends in
Ecology and Evolution, 14: 433-437.
Hu, S., F.S. III Chapin, M.K. Firestone, C.B. Field and N.R.
Chiariello (2001) Nitrogen limitation of microbial decomposition in a grassland under elevated CO2. Nature, 409:
188-191.
Intergovernmental Panel on Climate Change (IPCC) (2001)
IPCC Third Assessment Report-Climate Change 2001. The
Scientific Basis Technical Summary, Geneva.
Intergovernmental Panel on Climate Change (IPCC) (1996)
The science of Climate Change, 1996. In: J.T. Houghton, L.G.
Meira Filho, B.A. Callander, N. Harris, A. Kattenberg, andMaskell, K., eds., Cambridge Univ. Press, Cambridge.
Jamaludheen, V. and B.M. Kumar (1999) Litter of nine multipurpose trees in Kerala, India- variations in the amount, quality, decay rates and release of nutrients. Forest Ecology andManagement, 115: 1-11.
Johnson, K.G., K.A. Vogt, H.J. Clark, O.J. Schmitz and D.J.
Vogt (1996) Biodiversity and the productivity and stability of
ecosystems. Trends in Ecology and Evolution, 11: 372-377.
Johnson, D.W., R.B. Thomas, K.L. Griffin, D.T. Tissue, J.T.
Ball, B.R. Strain and F.T. Walker (1998) Effects of carbon dioxide and nitrogen on growth and nitrogen uptake in ponderosa and loblolly pine. Journal of Environmental Quality, 27:
414-425.
Kampichler, C., E. Kandeler, R.D. Bardgett, T.H. Jones and L.J.
Thomson (1998) Impact of elevated atmospheric CO2
concentration on soil microbial biomass and activity in acomplex, weedy field model ecosystem. Global Change
Biology, 4: 335-346.
Karnosky, D.F., B. Mankovska, K. Percy, R.E. Dickson, G.K.
Podila, J. Sober, A. Noormet, G. Hendry, M.D. Coleman, M.
Kubiske, K.S. Pregitzer and J.G. Isebrands (1999) Effects oftropospheric O3 on trembling aspen and interaction with CO2.
Results from and O3-gradient and a FACE experiment. Water,
Air and Soil Pollution, 116: 311-312.
Kasurinen, A., H.-S. Helimisaari and T. Holopainen (1999) The
influence of elevated CO2 and O3 on fine roots and mycorrhizas of naturally growing young Scots pine trees during three
exposure years. Global Change Biology, 5: 771-780.
Kaye, J.P., and S.C. Hart (1997) Competition for nitrogen between plants and microorganisms. Trends in Ecology and
Evolution, 12: 139-143.
Kemp, P.R., D.G. Waldeaker, C.E. Owensby, J.F. Reynolds and
R.A. Virginia (1994) Effects of elevated CO2 and nitrogen
fertilization pretreatments on decomposition of tall grassprairie leaf litter. Plant and Soil, 165: 115-127.
Koch, G.W. and H.A. Mooney (1996) Response of terrestrial
ecosystems to elevated CO2: a synthesis and summary. In:
G.W. Koch and H.A. Mooney, eds., Carbon Dioxide andTerrestrial Ecosystem. Academic Press, San Diego,
pp.415-429.
Koricheva, J., S. Larsson, E. Haukioja and M. Keinänen (1998)
Regulation of woody plant secondary metabolism byresource availability: hypothesis testing by means ofmeta-analysis. Oikos, 83: 212-226.
Körner, C. (2000) Biosphere responses to CO2 enrichment.
Ecological Applications, 10: 1590-1619.
Kumar, B.M. (2003) Impact of climate change on Indian agricultural and natural resources scenarios. In: B.M. Kumar, P.O.
Nameer and L.C. Babu, eds., Natural Resource Management: changing scenarios and shifting paradigms. Kerala
Agricultural University, Thrissur, Kerala, India, pp.66-76.
Kumar, B.M. and J.K. Deepu (1992) Litter production anddecomposition dynamics in moist deciduous forests of theWestern Ghats in Peninsular India. Forest Ecology andManagement, 50: 181-201.
Kumar, K. and K.M. Goh (2000) Crop residues and management practices: effects on soil quality, soil nitrogen dynamics,
crop yield and nitrogen recovery. Advances in Agronomy, 68:
197-319.
Lal, M., T. Nozawa, S. Emori, H. Harasawa, K. Takahashi, M.
Kimoto, A. Abe-Ouchi, T. Nakajima, T. Takemura and A.
Numaguti (2001) Future climate change: Implications for
Indian summer monsoon and its variability. Current Science,
81 (9): 1196-1207.
Lamborg, M.R., R.W.F. Hardy and E.A. Paul (1983) Microbial
effects. In: E.R. Lemon, ed., CO2 and Plants: The Responseof Plants to Rising Levels of Atmospheric Carbon Dioxide,
Westview Press, Boulder, CO, pp.131-176.
Lavelle, P., D. Bignell and M. Lepage (1997) Soil function in achanging world: the role of invertebrate ecosystem engineers.
European Journal of Soil Biology, 33: 159-193.
Lindroth, R.L., B.J. Kopper, F.J. Parsons, J.G. Bockhein, D.F.
Karnosky, G.R. Hendrey, K.S. Pregitzer, J.G. Isebrands and J.
Sober (2001) Consequences of elevated carbon dioxide andozone for foliar chemical composition and dynamics in trembling aspen (Populus tremuloides) and paper birch (Betula
papyrifera). Environmental Pollution, 115: 395-404.
Lippert, M., K. Steiner, T. Pfirrmann and H.D. Payer (1997)
Assessing the impact of elevated O3 and CO2 on gas exchange characteristics of differently K supplied clonal
Norway spruce trees during exposure and the following season. Trees Structure and Functions, 11: 306-315.
Luo, Y. and J.F. Reynolds (1999) Validity of extrapolating field
CO2 experiments to predict carbon sequestration in natural
ecosystems. Ecology, 80: 1568-1583.
Luan, J., Y. Luo and J.F. Reynolds (1999) Responses of a loblolly pine ecosystem to CO2 enrichment: a modeling analysis.
Tree Physiology, 19: 279-287.
Mackensen, J. and J. Bauhus (1999) The decay of coarse woody
debris. Technical Report No. 6. National Carbon Accounting
System. Australian Green House Office, Canberra, 47p.
McBride, J.R. and P.R. Miller (1999) Assessment of ecologicalrisks and implications for policy and management in the San
Bernardino Mountains. In: P.R. Miller and J.R. McBride, eds.,
Oxidant Air Pollution Impacts in the Montane Forests ofSouthern California, Springer, New York, pp.397-404.
McGuire, A.D., J.M. Melillo and L.A. Joyce (1995) The role ofnitrogen in the response of forest net primary production toelevated atmospheric carbon dioxide. Annual Review of Ecology and Systematics, 26: 473-503.
McClaugherty, C.A. and B. Berg (1987) Cellulose, lignin andnitrogen concentrations as regulating factors in late stages offorest litter decomposition. Pedobiologia, 30: 101-112.
Melillo, J.M, J.D. Aber and J.F. Muratore (1982) Nitrogen andlignin control of hardwood leaf litter decomposition dynamics. Ecology, 63: 621-626.
Mingkui, C. and F.I. Woodard (1998) Dynamic responses ofLitter Decomposition Under Elevated CO2 Levels
terrestrial ecosystem carbon cycling to global climaticchange. Nature, 393: 249-252.
Mooney, H.A., J. Canadell, F.S. Chapin, J.R. Ehleringer, C.
Körner, R.E. McMurtie, W.J. Parton, L.F Pitelka and E.D.
Schulze (1999) Ecosystem physiology response to globalchange. In: B. Walker, W. Steffen, J. Canadell and I. Ingram,
eds., The terrestrial Biosphere and Global ChangeImplications for Natural and Managed Ecosystems.
Cambridge University Press, Cambridge, pp.141-189.
Norby, R.J. and M.F. Cotrufo (1998) Global change: A questionof litter quality. Nature, 396: 17-18.
Norby, R.J., M.F. Cotrufo, P. Ineson, E.G. O’Neill and J.G.
Canadell (2001) Elevated CO2, litter chemistry and decomposition: a synthesis. Oecologia, 127:153-165.
Olszyk, D.M., M.G. Johnson, D.L. Phillips, R.J. Seidler, D.T.
Tingey and L.S. Watrud (2001) Interactive effects of CO2
and O3 on a pine plant/litter/soil mesocosm. Environmental
Pollution, 115: 447-462.
O’Neill, E.G. (1994) Response of soil biota to elevated atmospheric carbon dioxide. Plant and Soil, 165: 55-65.
Peñuelas, J. and M. Estiarte (1998) Can elevated CO2 affectsecondary metabolism and ecosystem function? Trends in
Ecology and Evolution, 13:20-24.
Pérez-Soba, M., T.A. Dueck, G. Puppi and J.C. Kuiper (1995)
Interactions of elevated CO2, NH3 and O3 on mycorrhizal
infection, gas exchange and N metabolism in saplings of
Scots pine. Plant and Soil, 176: 107-116.
Rasmussen, L., C. Beire and A. Bergstedt (2002) Experimental
manipulations of old pine forest ecosystems to predict thepotential tree growth effects of increased CO2 and temperature in a future climate. Forest Ecology and Management,
158: 179-188.
Rogers, H.H., G.B. Runion and S.V. Krupa (1994) Plant
responses to atmospheric CO2 enrichment with emphasis on
roots and the rhizosphere. Environmental Pollution, 83:
155-189.
Ross, D.J., K.R. Tate, P.C.D. Newton and H. Clarck (2003)
Carbon mineralization in an organic soil, with and without
added grass litter, from a high CO2 environment at a carbon
dioxide spring. Soil Biology and Biochemistry, 35:
1705-1709.
Runion, G.B., E.A. Curl, H.H. Rogers, P.A. Backman, R.
Rodriguezcabana and B.E. Helms (1994) Effects of free-air
CO2 enrichment on microbial populations in the rhizosphereand phyllosphere of cotton. Agricultural and Forest
Meteorology, 70: 117-130.
Rygiewicz, P.T., K.J. Martin and A.R. Tuininga (2000) Morphological diversity of ectomycorrhizas on Douglas -fir
(Pseudostuga menziesii Mirb. Franco) seedling grown underelevated atmospheric CO2 and temperature. Oecologia, 124:
299-308.
Salonius, P.O. (1981) Metabolic capabilities of forest soil
microbial populations with reduced species diversity. Soil
Biology and Biochemistry, 13: 1-10.
Seneviratne, G. (2000) Litter quality and nitrogen release intropical agriculture: a synthesis. Biology and Fertility of Soils,
31:60-64.
Sowerby, A., H. Blum, T.R.G. Gray and A.S. Ball (2000) The
decomposition of Lolium perenne in soil exposed to elevated
CO2: comparisons of mass loss of litter with soil respiration91

and soil microbial biomass. Soil Biology and Biochemistry,
32: 1359-1366.
Staddon, P.L., A.H. Fitter and J.D. Graves (1999) Effect of elevated atmospheric CO2 on mycorrhizal colonization, external
mycorrhizal hyphal production and phosphorus inflow in
Plantago lanceolata and Trifolium repens in association withthe arbuscular fungus Glomus mossae. Global Change Biology, 5: 347-358.
Swift, M.J., O.W. Heal and J.M. Anderson (1979) Decomposition in terrestrial ecosystems. Studies in Ecology 5. Blackwell Scientific, Oxford.
Tateno, M. and F.S. Chapin III. (1997) The logic of carbon andnitrogen interactions in terrestrial ecosystems. American
Naturalist, 149: 723-744.
Temperton, V.M., S.J. Grayston, G. Jackson, C.V.M. Barton, P.
Millard and P.G. Jarvis (2003) Effects of elevated carbon
dioxide concentration on growth and nitrogen fixation in
Alnus glutinosa in a long-term field experiment. Tree Physiology, 23: 1051-1059.
Thomas, C.D., A. Cameron, E.E. Green, M. Bakkens, L.J.
Beaumont, Y.C. Collingham, B.F.N. Erasmus, M.F. deSiqueira, A. Grainger, L. Hannah, L. Hughes, B. Huntley,
A.S. van Jaarsveld, G.F. Midgley, L. Miles, M.A. OrtegaHuerta, A.T. Peterson, O.L. Phillips and S.E. William (2004)
Extinction risk from climate change. Nature, 427: 145-148.
Thönnissen, C., D.J. Midmore, J.K. Ladha, D.C. Olk and U.
Schmidhalter (2000) Legume decomposition and nitrogen
release when applied as green manure to tropical vegetableproduction systems. Agronomy Journal, 92(2): 253-260.
Van Breemen, N., N. Jenkins, R.F. Wright, W.J. Arp, D.J.
Beerling, F. Berendse, C. Beier, R. Collins, D. van Dam, L.
Rasmussen, P.S.J. Verburg and M.A. Wills (1998) Impacts ofelevated carbon dioxide and temperature on a boreal forest
ecosystem (CLIMEX project). Ecosystems, 1: 345-351.
Vucetich, J.A., D.D. Reed, A. Breymeyer, M. Degorski, G.D.
Mroz, J. Solon, E. Roo-Zielinska and R. Noble (2000) Carbon pools and ecosystem properties along a latitudinal gradient in northern Scots pine (Pinus sylvestris) forests. Forest
Ecology and Management, 136: 135-145.
Walker, R.F., D.R. Geisinger, D.W. Johnson and J.T. Ball
(1995) Enriched atmospheric CO2 and soil P effects on
growth and ectomycorrhizal colonization of juvenile ponderosa pine. Forest Ecology and Management, 78: 207-215.
Wang, J.G. and L.R. Bakken (1997) Competition for nitrogen
during mineralization of plant resides in soil: microbial response to C and N. Soil Biology and Biochemistry, 29:
163-170.
Wardle, D.A., K.I Bonner and K.S. Nicholson (1997) Biodiversity and plant litter: experimental evidence, which does notsupport the view that enhanced species richness, improves
ecosystem function. Oikos, 79:247-258.
Zak, D.R., K.S. Pregitzer, J.S. King and W.E. Holmes (2000)
Elevated atmospheric CO2, fine roots and the response of soil
microorganisms: a review and hypothesis. New Phytologist,
147: 210-222.
Zak, D.R., K.S. Pregitzer, P.S. Curtis, J.A. Teeri, R. Fogel and
D.L. Randlett (1993) Elevated atmospheric CO2 and feedback between carbon and nitrogen cycles. Plant and Soil,
151: 105-117.

(Received 22 October 2004, Accepted 15 February 2005)